Canon XSi/450D

XSi AE/AF lock

Using AF Lock on the Canon XSi/450D can be trickier than using AE Lock.

When you press the shutter button halfway on a digital camera, both exposure and focus are locked simultaneously. This is usually, but not always, a good thing. We’ve all had those photos where the foreground in a scene looks great and the sky is totally washed out!

Metering with AE lock

Sometimes it’s preferable to lock exposure independently from focus. This can be accomplished using auto exposure lock (not all digital cameras have this useful feature).

Using AE lock on the XSI/450D

On the XSi/450D, focus first. Next, aim the camera where you want to lock exposure and press the (*) button on the top, back of the camera. Exposure is locked. Then, recompose and take the picture. [page 88 in the manual]. If you’re doing a close-up shot, watch the depth-of-field because recomposing could have an undesirable effect on the subject.

Focusing with AF lock a bit trickier

When the (*) is set to AE lock, the camera still locks focus and exposure together when half-pressing the shutter. However, this is not the case when the (*) is set to AF lock, where the camera locks focus when that button is pressed.

Because I take a lot of wildlife photos, I decided to switch the (*) button to AF lock [camera manual page #158: C.Fn-10 shutter/AE lock button]. I didn’t realize the camera no longer focused when half-pressing the shutter button.

No wonder my photos weren’t properly focused.

I should have known though, because there was no audible focus lock sound and no focus confirmation light. What did show in the viewfinder and on the LCD was a *. I now realize it was a reminder to hit the (*) button to focus.

Always something new to learn in digital photography.


  • Hi Gail.  Thanks so much for the reply.  I shoot wildlife, birds mostly, usually in Aperture Priority mode with One Shot AF.  Lately I’ve been shooting little dark birds perched on twigs sticking out of the snow.  As you can imagine, I use spot metering.  I aim for what I consider to be a mid-tone on the bird.  But boy, if the exposure doesn’t lock, and that metering spot hits the snow when I re-frame, my shot is garbage.  So yes, you’re absolutely right, page 88 is for me.  I was just hoping that one of the custom functions could be set such that the exposure (and focus) would lock on the half press by default.  But alas, I have to tell it to lock the exposure explicitly every time with the * button.  Of course if I switch CF10 to Mode 1, then yes, the half press does lock AE by default, but then I have to use * to focus, and it’s still a two-button solution.  Unable to find a way to have half press lock both (focus and exposure) I was sort of hoping there might be a way to have * lock both.  That way * would be a sort of “target select” button which would set focus and exposure, and when I released it, I could trip all the frames I pleased, re-framed as I pleased, even if I let up the half press.  I’m kind of beginning to think this is just my own personal fantasy.  I think I’m going to have to learn to use my opposable thumb, just as the great Canon designed me, in Chapter 4 Verse 88.  🙂  Thanks again, Gail!

  • Jamie, perhaps I’m misunderstanding you, but here’s what I’ve found to take multiple shots when using AE Lock. AE Lock remains activated for only four seconds. On page 88, #1, there is an icon that looks like a small clock with a 4 next to it, which indicates the time in seconds. (also see page 4, Conventions Used in the Manual)

    To keep AE Lock on for longer than four seconds, keep the shutter button half pressed even if you need to half press it again to refocus. The * stays on.

  • I wish there were a way to move the normal function of the half press entirely to the * button.  I like focusing with the * button but then it no longer locks the AE, and the first time I let up the shutter (I find it virtually impossible to “half release”) I’ve lost the exposure.  I see that the fit of the * button into the AF & AE locking scheme can be customized, but none of the available options seems to prevent a second half press from changing one or the other (focus or exposure).  Is my only option to go full manual on one of them (Manual Exposure or Manual Focus) with the other automated on the * button?  Either sounds like a rather severe speed penalty.  Is it so odd to want to be able to get a second shot without changing either?  And if not, why can’t I figure out how to do it?